Cooling steam control method for combined cycle power generation plants

ABSTRACT

The present invention is characterized in controlling the amount of cooling steam supplied to high temperature parts  8  in accordance with the dynamic characteristics of exhaust heat recovery boiler  2  when the load on gas turbine  1 C is changing, in a gas turbine steam cooling system provided with an exhaust heat recovery boiler  2  for operating gas turbine  1 C and generating steam using the exhaust heat from gas turbine  1 C, the combined cycle plant being controlled by driving the steam turbines  3,4  by directing a steam line from exhaust heat recovery boiler  2  to steam turbines  3,4,  cooling high temperature parts  8  of gas turbine  1 C by branching the steam line from exhaust heat recovery boiler  2,  and returning the cooled steam to the steam line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control method employed in a combinedcycle electric power generating plant, for controlling the amount ofsteam employed when using cooling steam to cool high temperature partssuch as the combustion chamber of a gas turbine, and for controllingsteam pressure in an exhaust heat recovery boiler which generates steamusing exhaust heat gases.

2. Description of the Related Art

There are conventionally known systems for use in a combined cycleelectric power generating plant employing gas turbines in which exhaustheat from the gas turbine is supplied to an exhaust heat recoveryboiler, steam is generated inside a drum in the boiler, and the steamturbine is operated using this steam, with cooling subsequently takingplace along the gas turbine's high temperature parts (combustionchamber, etc.). The method for controlling the steam line in thisarrangement is a significant problem, however.

In the case where a portion of the intermediate pressure steam in anexhaust heat recovery boiler is supplied as cooling steam to thecombustion chamber of a gas turbine, Japanese Patent Application, FirstPublication No. Hei 9-13917 discloses a method for adjusting the amountof intermediate pressure steam generated. In this method, the amount ofintermediate pressure steam generated is increased by tightening ahigh-pressure governor valve in the steam turbine.

U.S. patent application Ser. No. 5,471,832 discloses a method forcontrolling the cooling steam supplied to a gas turbine in the casewhere one combined cycle electric power generating plant is combinedwith another combined cycle electric power generating plant.

Japanese Patent Application No. Hei 8-285115 (Device and Method forControlling Gas Turbine Steam Cooling System) submitted by the currentapplicants discloses a method for detecting the steam temperature at theexit of the high temperature parts of the gas turbine, and controlling acontrol valve provided on the entrance side of the high-temperatureparts, so that the aforementioned detected temperature achieves apredetermined value.

In addition, Japanese Patent Application No. Hei 8-289907 (Gas TurbineSteam System), also submitted by the present applicants, discloses asystem provided with a pressure difference detecting device for the heatexchanger for the high temperature parts in the gas turbine; aprotective control valve provided to the steam line which passes throughthe heat exchanger and goes to the steam condenser of the high andintermediate pressure steam turbines; and a temperature controllingdevice for receiving the signal output from the pressure differencedetecting device, and sending a control signal for controlling thetemperature of the heat exchanger to the protective control valve.

The conventional control methods described above have the followingproblems, however.

(1) In the case of a method for controlling the amount of steam suppliedto the high temperature parts based on the temperature of the steam atthe exit of the high temperature parts, sufficient cooling of the partsis not possible if the temperature of the steam at the exit becomes toohigh. Conversely, if the steam temperature becomes too low, thetemperature of the steam supplied to the steam turbine falls, and theefficiency of the steam turbine decreases. Accordingly, the temperatureof the steam at the exit of the high pressure parts must be controlledto an appropriate set value. However, if the value in the case where theload on the gas turbine is constant is designated as this set value,then the steam temperature value will not be appropriate if the load onthe turbine changes.

(2) The amount of steam required to cool the high temperature partsincreases when the load on the gas turbine rises. However, inconventional combined plants where plant efficiency is emphasized, thereis little leeway for ensuring this additional quantity of steam.

(3) When the steam pressure in an exhaust heat recovery boiler becomesextremely high, a safety valve operates to protect the exhaust heatrecovery boiler, releasing steam into the atmosphere. Some of thisrelease includes the steam for cooling the high temperature parts,however, so that a shortage of cooling steam results.

The present invention was conceived in consideration of theabove-described circumstances, and has as its first objective, theprovision of a steam temperature control method which operates inresponse to changes in the load on the gas turbine. The presentinvention has as its second objective, the provision of a method forcontrolling steam pressure which is capable of accumulating steam in theexhaust heat recovery boiler when the gas turbine is operating under aconstant or falling load. Finally, the present invention has as itsthird objective, the provision of a method for controlling steamtemperature which operates in response to pressure when the steampressure in the exhaust heat recovery boiler becomes extremely high.

SUMMARY OF THE INVENTION

In order to achieve the above-stated objectives, the present inventionprovides a cooling steam system control method for a combined cycleplant, wherein, in a gas turbine steam cooling system provided with anexhaust heat recovery boiler for operating a gas turbine and generatingsteam using the exhaust heat from the gas turbine, the combined cycleplant is controlled by driving the steam turbine by directing a steamline from the exhaust heat recovery boiler to the steam turbine, coolingthe high temperature parts of the gas turbine by branching the steamline from the exhaust heat recovery boiler, and returning the cooledsteam to the steam line; the cooling steam system control methodcharacterized in controlling the amount of cooling steam supplied to thehigh temperature parts in accordance with the dynamic characteristics ofthe exhaust heat recovery boiler when the load on the gas turbine ischanging.

The present invention also provides a cooling steam system controlmethod characterized in the provision of a load characteristicsdetecting device and in the provision of a steam temperature detectingdevice to the steam line at the exit of the high temperature components,wherein:

a static characteristics value signal for the steam temperature in thehigh temperature parts corresponding to load is set from the loadcharacteristics detecting device via a function device, when the load onthe gas turbine is static, the signal is introduced to a subtractingdevice after passing through a correcting circuit, and the cooling steamflow amount is adjusted by means of a control valve provided at theentrance or exit side of the high temperature parts on the steam line,via a temperature controlling device; and

a dynamic characteristics signal for the steam temperature in the hightemperature parts corresponding to a changing load is set in thecorrecting circuit from the load characteristics detecting device viathe function device, and the extent to which the control valve is openis adjusted, when the lord on the gas turbine is changing.

In this case, it is preferable to set the static characteristics valuetemperature when the load on the gas turbine is static to beapproximately the same as the temperature of the steam at the exit ofthe steam line reheating device inside the exhaust heat recovery boiler.

It is also possible to provide a steam pressure detecting device to theline for guiding the steam to the steam turbine on returning process thesteam cooling the high temperature parts to the steam line of theexhaust heat recovery boiler; and to perform control based on thepressure signal from this detecting device by employing a pressurecontrolling device so that the steam pressure at the source forgenerating the cooling steam in the low load region of the gas turbineincreases.

In addition, it is also possible to provide a pressure detecting deviceat the source for generating cooling steam in the exhaust heat recoveryboiler; operate a pressure controlling valve provided to the downstreamline by sending the signal output from this pressure detecting device toa pressure controlling device, and then sending the signal from here tothe pressure control valve; and operate a temperature control valve inaccordance with the signal from the pressure controlling device when thepressure is high, by sending the signal output from the pressuredetecting device to a high value selecting device provided between thetemperature controlling devices for the high temperature parts.

BRIEF EXPLANATION OF THE FIGURES

FIG. 1 is an example of a schematic system diagram for a steam line fora combined cycle plant according to the present invention.

FIG. 2 is a schematic system diagram of the control line and structureof a combined cycle plant showing an embodiment of a present invention.

FIG. 3 is a graph showing the static characteristics between the steamtemperature and load on the gas turbine.

FIG. 4 is a diagram of the correlation between load and the steamtemperature set value in the case where the load on the gas turbine haschanged from the time the value was set.

FIG. 5 is a diagram of the correlation between the change in load andthe change in steam temperature, in the correcting circuit.

FIG. 6 is graph comparing the control set values for the pressure of theintermediate pressure drum in the present invention and the conventionalart.

PREFERRED EMBODIMENTS OF THE PRESENT EMBODIMENTS

An example of a schematic system diagram for a steam line in a combinedcycle plant is shown in FIG. 1. Gas turbine main body 1 is formed ofcombustion chamber 1A for combusting fuel; compressor 1B for formingcompressed air; and gas turbine 1C which operates by receiving a supplyof combustion gas. Gas turbine 1C operates to drive electric powergenerator G, thereby generating electric power. The exhaust gasgenerated at gas turbine 1C is sent to exhaust heat recovery boiler 2.The heat from this exhaust gas is used to generate steam from the watersupplied to intermediate pressure drum 9 and high pressure drum 5 inexhaust heat recovery boiler 2. The steam generated at high pressuredrum 5 is supplied to high pressure steam turbine 3 after passingthrough high pressure heating device 6 provided to the downstreamportion of the steam line. The steam generated at intermediate pressuredrum 9 is supplied to intermediate pressure steam turbine 4 viaintermediate pressure heating device 10 and reheating device 7 providedto the downstream portion of the steam line. The steam exiting highpressure steam turbine 3 converges at the entrance to reheating device7. Meanwhile, although not shown in the figures, the steam exitingintermediate pressure turbine 4 enters a low pressure steam turbine, isreturned to water at a steam condenser, and then employed in the steamgenerating drum.

As shown in FIG. 2, in the embodiments of the present invention, steamexiting intermediate pressure heating device 10 branches, traveling viatemperature control valve 12 and high temperature part 8 (i.e.,combustion chamber 1A or gas turbine 1C of gas turbine main body 1; mayalso include a heat exchanger for the high temperature parts), toconverge with the steam exiting reheating device 7. The signal outputfrom exit-steam-temperature detecting device 15 of high temperature part8 is sent to temperature control valve 12 via temperature controllingdevice 11, to regulate the extent to which the valve is open. Note thata drum for generating low pressure steam is disposed inside exhaust heatrecovery boiler 2, and a low pressure steam turbine is disposed to thesteam turbine. However, an explanation of these in the design of thepresent invention is omitted as unnecessary.

In conventional control methods, the extent to which temperature controlvalve 12 is open is controlled according to the signal output fromexit-steam-temperature detecting device 15 for high temperaturecomponent 8, with temperature control valve 12 operated so that thetemperature of the steam exiting high temperature component 8 becomes apredetermined value. As discussed above, however, it is not possible tocontrol the temperature of the steam exiting high temperature component8 to the appropriate set level when the load on gas turbine 1C ischanging.

In contrast, in the present invention, a load signal 1S from gas turbine1C is sent to subtracting device 20 via function device 16 andcorrecting circuit 18. At the same time, the signal output fromexit-steam-temperature detecting device 15 for high temperature part 8is sent to subtracting device 20, subtracted, and relayed to temperaturecontrolling device 11. Temperature controlling device 11 and subtractingdevice 20 together form the temperature controlling means.

As shown in FIG. 3, a static characteristics function for the steamtemperature and the gas turbine load are set at function device 16. Loadsignal 1S outputs signal 16S for the steam temperature staticcharacteristics setting value. In other words, signal 16S is output sothat the exit steam temperature at high temperature part 8 isapproximately the same as the exit steam temperature of reheating device7.

When load signal 1S for gas turbine 1C is constant, correcting circuit18 generates signal 16S for the steam temperature static characteristicssetting value as the output signal without modification. Subtractingdevice 20 receives this signal 16S and the signal output fromexit-steam-temperature detecting device 15, and sends a deviation signalto temperature controlling device 11. The signal output from subtractingdevice 20 is received at temperature controlling device 11, and acontrol signal is relayed to temperature control valve 12. As a result,the valve opening of control valve 12 is adjusted in accordance with thecontrol signal, and the exit steam temperature at high temperaturecomponent 8 is set.

When load signal 1S of gas turbine 1C varies, correcting circuit 18receives signal 16S for the steam temperature static characteristicssetting value, adds a correction based on the correspondingcharacteristics of exhaust heat recovery boiler 2, and outputs thesignal.

FIG. 4 is a diagram of the correlation between time, steam temperatureand the load state on gas turbine 1C. The static characteristics valueis expressed when there is no load applied. When a load is applied, theamount of fuel charged to gas turbine 1C increases and the steamtemperature rises. In addition, the temperature of the steam exitingafter cooling gas turbine 1C also increases. Further, the temperature ofthe exhaust gas from gas turbine 1C at this time also rises.Accordingly, the thermodynamic characteristics (dashed line) of exhaustheat recovery boiler 2 which employs the aforementioned expelled gas aredetected and relayed as an output signal to correcting circuit 18, wherecorrection is then performed between this signal and the steamtemperature static characteristics value which is the output signal fromfunction device 16. This corrected signal 18S for the exit steamtemperature set value is input to subtracting device 20. The detectionsignal at exit steam temperature detecting device 15 for hightemperature component 8 is also input to subtracting device 20. Thesignal subtracted here is then relayed to temperature controlling device11. As a result, the degree to which temperature control valve 12 isopen is controlled, thereby controlling the steam flow amount.

FIG. 5 is a diagram showing the correlation between load and temperatureat correcting circuit 18. This figure shows the dynamic characteristicsof temperature accompanying the load on gas turbine 1C. This figureshows that when the load changes, time is required until the temperaturereaches a value corresponding to that load. In other words, the timedelay from when the quantity of fuel charged to gas turbine 1C changesuntil the exit steam temperature at reheating device 7 changes, whichaccompanies the absorption of heat by reheating device 7, is correctedby correcting circuit 18. Ideally, the design of correcting circuit 18for correcting this delay would have a dynamic characteristics model ofexhaust heat boiler 2. However, in order to simplify the adjustingmethod, this objective can be sufficiently met simply by designating atemporary delay as a required element. Experimentally, 10 to 15 minutesare required for the delay. Note that the signal fromexit-steam-temperature detecting device 13 at reheating device 7 may beused in place of correcting circuit 18.

The second objective of the present invention relates to accumulatingsteam in exhaust heat recovery boiler 2 when the load on gas turbine 1is constant or falling. This embodiment will now be explained withreference to FIG. 6.

In the conventional combined plant, the steam pressure is determined bythe amount of flow into steam turbines 3,4 in comply with the course ofoperations, and is not particularly controlled. In other words, theamount of steam taken in by steam turbines 3,4 and the amount of steamgenerated from exhaust heat recovery boiler 2 are essentially the same.Accordingly, in this invention, a pressure control valve 22 is providedto the ejection system for the cooling steam from exhaust heat recoveryboiler 2, the steam pressure in exhaust heat recovery boiler 2 in thelow load region is controlled to be a high value so that steamaccumulates in exhaust heat recovery boiler 2, and the steam pressure inexhaust heat recovery boiler 2 is controlled based on the entrancepressure at intermediate pressure steam turbine 4.

In other words, as shown in FIG. 2, a pressure detecting device 21 isprovided at the entrance to intermediate pressure steam turbine 4 fordetecting the entrance pressure. The signal from pressure detectingdevice 21 is sent to function device 23. The signal converted atfunction device 23 is sent to subtracting device 24, subtracted from thesignal from pressure detecting device 17 in intermediate pressure drum4, and then sent to pressure controlling device 19, to operate pressurecontrol valve 22. Pressure control valve 22 is positioned on the steamline to exhaust heat recovery boiler 2 at a point downstream to wherethe line branches off to high temperature part 8.

In low load regions, such as when the load on gas turbine 1C is fallingor constant, the heat input energy to high temperature part 8 is small,i.e., the load for cooling high temperature part 8 is small.Accordingly, the opening of temperature control valve 12 which iscontrolled via temperature controlling device 11 is small, and theamount of steam generated in intermediate pressure drum 9 is insufficient excess with respect to the amount of steam required forcooling. In this case, the steam pressure inside the intermediatepressure system from intermediate pressure drum 9 to pressure controlvalve 22 is increased and intermediate pressure steam accumulates, bytightening pressure control valve 22 of intermediate pressure drum 9with respect to the entrance pressure at intermediate pressure steamturbine 4 based on the function which has been set to a high value atpressure controlling device 19 as shown in FIG. 6.

Conversely, when the load on gas turbine 1C becomes high, hightemperature part 8 reaches a high temperature. In response, steamaccumulated in the intermediate pressure system is supplied.Accordingly, steam is smoothly supplied without an insufficiencyoccurring in the quantity of cooling steam, even when the load on gasturbine 1 is rising. When a significant amount of intermediate pressuresteam flows into high temperature part 8, the amount of cooling steamrequired increases. In this case, however, pressure control valve 22 ofintermediate pressure drum 9 is closed due to control by pressurecontrolling device 19, and the supply of intermediate pressure steam tothe steam line via reheating device 7 is halted.

In the present invention, the pressure of intermediate pressure steam iscontrolled according to the entrance pressure at intermediate pressuresteam turbine 4. Accordingly, one steam turbine 3,4 is connected betweena plurality of gas turbines 1C and a plurality of exhaust heat recoveryboilers 2, and is effective when utilizing a typical combined cycleelectric power generating plant. In other words, in a plant of thistype, the amount of steam generated differs according to the number ofgas turbines 1C and exhaust heat recovery boilers 2 operating, with theentrance pressure varying in a similar manner accompanying this. In thepresent invention, however, the pressure of the intermediate pressuresteam can be maintained at a suitable level even in the case where thenumber of gas turbines 1C and exhaust heat recovery boilers 2 operatinghas changed.

An explanation will now be made of the present invention's thirdobjective relating to control in the case where the steam pressure inexhaust heat recovery boiler 2 rises significantly. In this case, asshown in FIG. 2, a pressure detecting device 17 is provided tointermediate pressure drum 9 which is the generating source for thecooling steam, this output signal is communicated to pressurecontrolling device 25, and input to high value selecting device 14 ascontrol signal 25S. control signal 11S from temperature controllingdevice 11 is also input to high value selecting device 14. This highvalue selecting device 14 controls temperature control valve 12.

Ordinarily, pressure control device 19 controls pressure control valve22 which regulates the amount of flow in the system which forms thesteam line extending to intermediate pressure steam turbine 4 viareheating device 7. In the event of an anomaly, however, such as amalfunction in pressure control valve 22 or the control circuit, whenthe output signal from pressure detecting device 17 of intermediatepressure drum 9 rises above high value H, this output signal passesthrough pressure controlling device 25 to become control signal 25Swhich is higher than control signal 11S from temperature controllingdevice 11, and is switched at high value selecting device 14. Controlsignal 25S is sent to temperature control valve 12, and the steam flowamount is increased by further opening valve 12 in response to thisinput. As a result, high temperature part 8 is cooled and operationscontinue while controlling the increase in steam pressure. Thus, even inthe case of an extreme increase in pressure in intermediate pressuredrum 4, steam is not released to the outside of the system, but rather aloss of steam is prevented by flushing into the steam line. Accordingly,a highly reliable closed steam circuit is formed.

Note that the present invention is not limited to the embodimentsexpressed here, but rather, includes variations and modificationsthereon, provided these do not depart from the spirit of the invention.

What is claimed:
 1. A cooling steam system control method for a combined cycle plant, wherein the system includes a gas turbine load characteristics detecting device a steam temperature detecting device connected to the steam line at the exit of the high temperature components, comprising the steps of: driving the steam turbine by directing a steam line from the exhaust heat recovery boiler to the steam turbine; cooling high temperature parts of the gas turbine by branching the steam line from the exhaust heat recovery boiler; returning the cooled steam to the steam line; and controlling the amount of cooling steam supplied to the high temperature parts in accordance with dynamic characteristics of the exhaust heat recovery boiler when the load on the gas turbine is changing, wherein the controlling step comprises the steps of, setting a static characteristics value signal for the steam temperature in the high temperature parts corresponding to load via a function device when the load on the gas turbine is static, wherein the signal is introduced to a subtracting device after passing through a correcting circuit, and the cooling steam flow amount is adjusted by means of a control valve provided at the entrance or exit side of the high temperature parts on the steam line via a temperature controlling device, and setting a dynamic characteristics signal for the steam temperature in the high temperature parts corresponding to a changing load in the correcting circuit from the load characteristics detecting device via the function device, wherein the extent to which the control valve is open is adjusted when the load on the gas turbine is changing.
 2. A cooling steam system control method for a combined cycle plant according to claim 1, further comprising the step of setting the static characteristics value temperature, when the load on the gas turbine is static, to be approximately the same as the exit steam temperature of the steam line after burning device inside the exhaust heat recovery boiler.
 3. A cooling steam system control method for a combined cycle plant according to claim 1, further comprising the steps of: guiding the steam to the steam turbine, during a return process, whereby the steam cools the high temperature parts to the steam line of the exhaust heat recovery boiler; and controlling, based on the pressure signal from the detecting device, by employing a pressure controlling device in order that the steam pressure at the source for generating the cooling steam in the low load region of the gas turbine increases.
 4. A cooling steam system control method for a combined cycle plant according to claim 1, further comprising the steps of: generating cooling steam in the exhaust heat recovery boiler; operating a pressure controlling valve which is provided to the downstream line by sending the signal output from the pressure detecting device to a pressure controlling device; sending the signal from the pressure controlling device to the pressure control valve; and operating a temperature control valve in accordance with the signal from the pressure controlling device when the pressure is high by sending the signal output from the pressure detecting device to a high value selecting device provided between the temperature controlling devices for the high temperature parts. 